1. Field of the Invention
The present invention relates to an electrode structure of a vanadium redox flow battery, and particularly to an electrode structure of a vanadium redox flow battery in which the graphite felt units are embedded in the flow channels of graphite polar plates.
2. The Prior Arts
With the rising environmental protection awareness and the upcoming era of high gasoline price, the energy-related companies and governments worldwide have been trying to develop the renewable green energies, which are produced by the ocean waves or tides, geothermal energy, wind energy, and solar cells, to replace the conventional petrochemical energies which will be run out eventually. Because the fluctuations of electrical energy produced by the renewable green energy are great, an auxiliary energy storage system is often needed to store excessive electrical energy or to stabilize the electrical current, so that the electrical energy can be stored when the electrical energy generation is abundant, and can be fed back to the electricity network when the electrical energy generation is not enough.
A redox flow battery is often used in an energy storage system. Just like the fuel batteries, the electrode of a redox flow battery is only involved in catalysis, but not involved in the reaction, and thereby the electrode would not be consumed or enlarged. The reactants in the sulfuric acid solutions are fed into the cell stack from the external tank, and electrochemically reacted to form the products with chemical energy, and then the electrochemical products are stored in the external tank. The stored chemical energy can be also converted into the electrical energy when the electrochemical products are released from the external tank to the cell stack for discharging. Therefore, the redox flow battery is very suitable to be used in the electrical charging or discharging.
The vanadium redox flow battery (VRB) has the advantages of fast response time, high performance/price ratio, flexible design, and a long cycle life, etc. The vanadium redox flow battery has received a lot of attention of the researchers, and is very suitable to be used as a large-scale energy storage device.
FIG. 1 is a schematic view showing a conventional vanadium redox flow battery. As shown in FIG. 1, the conventional vanadium redox flow battery includes a plurality of positive electrode plates 10, a plurality of negative electrode plates 20, a positive electrolyte 30, a negative electrolyte 40, a positive electrolyte external tank 50, and a negative electrolyte external tank 60. The positive electrolyte 30 and the negative electrolyte 40 are respectively stored in the external tank 50 and the external tank 60. At the sometime, the positive electrolyte 30 and the negative electrolyte 40 respectively pass through the positive electrode plates 10 and the negative electrode plates 20 via the positive connection pipelines and the negative connection pipelines to form the respective loops themselves indicated as the arrows shown in FIG. 1. Pumps (not shown) are often installed on the connection pipelines for continuously transporting the electrolytes to the electrode plates.
Moreover, a power conversion unit 90, e.g. a DC/AC converter, can be used in a vanadium redox flow battery, and the power conversion unit 90 is respectively electrically connected to the positive electrode plates 10 and the negative electrode plates 20 via the positive connection lines 70 and the negative connection lines 80, and the power conversion unit 90 also can be respectively electrically connected to an external input power source 92 and an external load 94 in order to convert the AC power generated by the external input power source 92 to the DC power for charging the vanadium redox flow battery, or convert the DC power discharged by the vanadium redox flow battery to the AC power for outputting to the external load 94.
Generally, the sulfuric acid solutions containing the vanadium ions in different oxidation states, namely the redox couples of V(IV)/V(V) and V(II)/V(III), are respectively served as the positive electrolyte 30 and the negative electrolyte 40, and the following electrochemical reaction will be performed:

The advantages of a vanadium redox flow battery includes:    (1) The size of the cell stack determines how much power can be charged and discharged in the vanadium redox flow battery. i.e. the surface area of the electrode, the number of the single cells; and the amount of electrolyte determines how much energy can be stored in the vanadium redox flow battery, and thereby the vanadium redox battery having large scale storage capacity can be flexibly installed.    (2) No phase transformation is involved, and thereby the cycle life of the vanadium redox flow battery is greatly prolonged.    (3) The deep-discharge damage to the battery can be prevented.    (4) The vanadium redox flow battery can be instantly charged and discharged.    (5) The electrolyte storages are stable, and thereby vanadium redox flow battery can be preserved over a long period of time because of its low self-discharge rate.    (6) The battery structure is simple, and thereby the maintenance is easy.    (7) The same active substance is utilized in the positive and negative electrodes, and there are no problems of cross-contamination when the active substance penetrates through the separator.
Accordingly, by using the vanadium redox flow battery, the problem of intermittent power generation of renewable energy can be solved, so that the uncertainty for supplying power to the electricity network using the renewable energy is improved. As present, the vanadium redox flow battery is applied to: (1) an electric power company for massively storing the electricity and balancing the load; (2) a mid-scale electricity user, factory, company, and building in the remote area for providing them electric power or emerging electric power systems; (3) a home user; and (4) an auxiliary energy storage equipment for renewable energy such as wind or solar energy.
However, the disadvantages of the above-mentioned conventional flow battery include the dead volume and concentration polarization of the electrolyte, which would cause the decrease of the efficiency of the electron transfer in a battery so that the energy efficiency is decreased. Therefore, there is a need for providing a vanadium redox flow battery in which the electrode has an increased reaction area, and an efficient charge transfer so that the electric current density can be increased, and the energy efficiency can be improved in order to solve the problems of the conventional flow battery described above.